An important step in the process of integrated circuit manufacturing is the processing of the semiconductor substrate in which active devices such as transistors and capacitors that comprise the integrated circuits are formed. The silicon substrate, known as a wafer, must be manufactured to extremely precise specifications and quality standards. As in any manufacturing industry, minimization of defects is an important consideration. Because the active devices that are formed on the silicon substrate are microscopic in size, any defect in the substrate, even on the molecular or atomic level, will decrease yield and therefore increase the cost of manufacturing integrated circuits.
In an effort to minimize defects in silicon substrates, great care is taken to provide an extremely clean and controlled environment throughout the processing of the substrates. Toward this end, the processing chamber is periodically cleaned by opening the chamber, and chamber parts are either cleaned and reinstalled or replaced. This preventive maintenance can expose the surfaces of the process chamber to ambient air and moisture, which can be sources of metal contamination. Also, in the case of an epitaxial silicon or polysilicon deposition chamber (epi or poly chamber), the chamber and platform surfaces are periodically "etched" to remove excess silicon that has accumulated during previous cycles of silicon deposition.
Defects in the wafer can occur as a result of contaminants such as metal particles being transferred to the wafer during handling of the wafer or during processing of the wafer. Metal impurities in contact with the wafer can then diffuse into the bulk of the wafer during high temperature processing. Such contaminants can later change the electrical properties of a device manufactured in the wafer, thus causing a faulty integrated circuit. The impurities can act as recombination centers and traps for electrons and holes and can have a great effect on leakage currents. Typically, the impurities are metals such as iron, nickel, cobalt, and copper.
During high temperature processing, metal impurities can diffuse through the crystalline structure of the silicon material into the bulk of the wafer. The migration of metal impurities through the silicon lattice structure of a semiconductor wafer depends on the solubility and diffusivity of the various metals. Solubility is a measure of the maximum impurity concentration which can be dissolved in thermal equilibrium in a sample at a given temperature. If the impurity concentration is higher than its solubility, the material is super-saturated. Diffusivity is a measure of how fast the impurity diffuses through the lattice at a certain temperature.
Processing at high temperatures causes high solubility of metal impurities. The solubility decreases during cool down. The behavior of the impurities during cool down depends heavily on their diffusion properties. For example, impurities with low diffusivity can be quenched-in as metastable, electrically active point defects in the bulk of the material during a relatively fast decrease in temperature.
Impurities with high diffusivity and high solubility (copper, cobalt, and nickel, for example) are called haze metals because they diffuse to the surface during cool down and form surface precipitates. Impurities with low diffusivity might not be able to reach the surface during cool down. Instead they form precipitates within the bulk of the wafer or are trapped in the silicon lattice as point defects.
Iron, nickel, copper, and cobalt, can all easily diffuse through the thickness of a wafer at temperatures higher than 1,000 C. Most of these metals can diffuse on the order of millimeters or centimeters during a one minute process at such high temperatures.
FIG. 1 shows examples of metal contamination on a silicon wafer 12 that is resting on a susceptor 14. Example A of FIG. 1 shows particles on the edge of the wafer 12 that may have been transferred from a contaminated container or wafer carrier such as a wafer box or load lock cassette. Example B of FIG. 1 shows particles on the wafer surface that can be transferred to the wafer from tools such as robot arms, vacuum tweezers, metrology tool chucks, or even from ambient air. Example C of FIG. 1 shows homogeneous surface contamination due to wet cleaning with contaminated chemicals. Example D of FIG. 1 shows particles on the susceptor present before installation or from contaminated wafers that have been previously processed on the susceptor. Examples E and F of FIG. 1 show metal silicides formed on the wafer surface (E) or susceptor surface (F) during heat treatment. Each of Examples A through F shows metal contamination that can diffuse into the bulk of the wafer at high processing temperatures, as illustrated by the arrows in FIG. 1.
A number of sources of contamination are possible. For example, direct contact between the silicon wafer and metal parts can transfer metal particles to the wafer. Chucks, tweezers, wands, robots, and general improper handling can result in direct contact of metal to the silicon wafer. Another possible source is a contaminated cleaning solution. Metal atoms with higher electronegativity than silicon and which are present in the cleaning solution or on the cleaning vessel will segregate on the wafer surface. Such metals could be iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), silver (Ag), platinum (Pt), gold (Au), and mercury (Hg), for example. At lower temperatures, the metals can evaporate from the wafer. At higher temperatures, however, it is more common to find metal silicides which act as a contamination source. At very high temperatures, the metals can diffuse into the bulk of the wafer.
It is highly desirable to avoid the introduction of metal contaminants into silicon processing chambers. In many cases, however, some metal contamination is unavoidable. This is especially true in the case of preventive maintenance or during installation of new parts in a processing chamber when the chamber is exposed to ambient air or moisture. New parts can have metal contaminant residual from machining or coating steps in the hardware manufacturing process. In situations where a processing chamber has been exposed to metal contaminants that can potentially be transferred to silicon substrates, it is desirable to remove as many contaminating particles from the chamber as possible before processing of production wafers begins. Current methods of cleaning processing chambers to minimize metal contamination of wafers require significant "down time" of the processing chamber. Also, current methods can have high material or operating costs (e.g. processing of "dummy" wafers and baking procedures) in addition to the lost costs of the chamber that is temporarily out of service.